The present invention relates to an AC generator mainly for use in an automobile.
In general, an AC generator used in an automobile is of three phase AC type and the output thereof is converted into a DC current by a rectifier in order to charge a battery. Since this type of generator is mounted on an automobile and the conditions for the output of the generator to be saturated in relation to the temperature varies, the resistance value of the field winding is not constant thereby resulting in a variation of the full output power of the generator. Further, the torque required for an engine to drive this type of generator, i.e. a driving torque, is related to the full output power of the generator, so that the driving torque varies when the full output power of the generator varies. Particularly, the rotational speed of the engine in a low speed range disadvantageously varies when the full output power of the generator fluctuates.
FIG. 1 is a circuit diagram of a conventional AC generator for an automobile, in which a reference numeral 1 designates a stator winding capable of generating an AC power, 2 a field winding, 3 a rectifying apparatus having a group of rectifier elements for rectifying the AC power from the stator winding, 4 a voltage control apparatus for controlling the output power of the stator winding by intermittently flowing an exciting current into the field winding 2, and 41 a power transistor of the voltage control apparatus 4. With this construction, when the generator is supplied with a preset voltage such as 13.5 V and delivers the full output power, the power transistor 41 is conducting fully and has a collector-emitter voltage (V.sub.CE (SAT)) of about 1 V.
Supposing a resistance value of the field winding 2 is R, the preset voltage is V, and the V.sub.CE (SAT) of the power transistor 41 is v, a field current I.sub.f can be expressed by I.sub.f =(V-v/R).
The field current I.sub.f is related to the V.sub.CE (SAT) of the power transistor and the resistance value R of the field winding. The following explanation will be made on the basis that I.sub.f is substantially in inverse proportion to R, because the fluctuation of V.sub.CE (SAT) is a negligible value.
The resistance value R varies when the temperature of the field winding 2 varies to thereby change the magneto-motive force of the field winding. As is well known, the full output power characteristics where the temperature of the field winding is the same as the atmospheric temperature is referred to as the output power characteristics in a cooled state. The full output power characteristics where the temperature of the field winding is increased to a saturated state due to the Joule heat of the conduction current thereof is referred to as the output power characteristics in a heated state. The output characteristics in the cooled and heated states are illustrated in FIG. 2 by dotted and solid lines, respectively. The difference between the output current characteristics in the cooled and heated states is mainly due to the change in the resistance value of the field winding. The difference of the output current in the cooled and heated states results in the different driving torques, as shown in FIG. 2, which accounts for the variation in the rotational speed of the engine.
An object of the present invention is to provide an AC generator for a vehicle which obviates the above described drawbacks of the conventional AC generator.